1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an in-plane switching mode LCD device which includes a heater line to achieve an improvement in reliability under low temperature conditions.
2. Discussion of the Related Art
With the progress of information-dependent society, the demand for various display devices has increased. To meet such a demand, efforts have recently been made to research flat panel display devices such as liquid crystal displays (LCDs), plasma display panels (PDPs), electro-luminescent displays (ELDs), vacuum fluorescent displays (VFDs), and the like. Some types of such flat panel display devices are being practically applied to various appliances for display purposes.
In particular, LCDs have been used as a substitute for cathode ray tubes (CRTs) in association with mobile image display devices because LCDs have advantages of superior picture quality, lightness, thinness, and low power consumption. Thus, LCDs are currently widely used. Various applications of LCDs are being developed in association with not only mobile image display devices such as monitors of notebook computers, but also monitors of TVs to receive and display broadcast signals, and monitors of laptop computers.
Successful application of such LCDs to diverse image display devices depends on whether or not the LCDs can realize desired high picture quality, including high resolution, high brightness, large display area, and the like, while maintaining desired characteristics of lightness, thinness, and low power consumption.
Such an LCD mainly includes a liquid crystal panel for displaying an image, and a driver for applying a drive signal to the liquid crystal panel. The liquid crystal panel includes a first substrate and a second substrate joined together in such a manner that a space is defined between the first and second substrates, and a liquid crystal layer sealed in the space between the first and second substrates.
The first substrate (TFT array substrate) includes a plurality of gate lines arranged in one direction while being uniformly spaced apart from one another, and a plurality of data lines arranged in a direction perpendicular to the gate lines while being uniformly spaced apart from one another. The first substrate also includes a plurality of pixel electrodes arranged in a matrix array at respective pixel regions each defined by an intersection between each gate line and each data line, and a plurality of thin film transistors (TFTs), each of which is switched on by a signal on an associated one of the gate lines, and transmits a signal on an associated one of the data lines to an associated one of the pixel electrodes.
The second substrate (color filter substrate) includes a black matrix layer for blocking incidence of light to a region other than the pixel regions, R, G, and B color filter layers for reproducing color tones, and a common electrode for reproducing an image.
The driving principle of the above-mentioned general LCD utilizes optical anisotropy and polarization of the liquid crystal. Since liquid crystal has a thin and elongated molecular structure, molecules thereof have an orientation in a certain direction. It is possible to control the orientation of liquid crystal molecules by intentionally applying an electric field to the liquid crystal molecules.
In accordance with such a control for the orientation of liquid crystal molecules, the arrangement of liquid crystal molecules is varied, so that the liquid crystal molecules exhibit optical anisotropy. Since light incident to the liquid crystal is refracted in the direction in which the liquid crystal molecules are oriented, image information is represented.
The LCD having the above-mentioned driving principle is called a “twisted nematic (TN) mode LCD”. Since such a TN mode LCD has a drawback of a narrow viewing angle, an in-plane switching (IPS) mode LCD has been developed to overcome the drawback of the TN mode LCD.
In the IPS mode LCD, a pixel electrode and a common electrode are formed on a first substrate at each pixel region of the first substrate such that the pixel electrode and common electrode extend parallel to each other while being horizontally spaced apart from each other to generate an in-plane electric field (horizontal field) . The liquid crystal layer is oriented in a certain direction by the in-plane electric field.
Meanwhile, LCDs perform display of an image in accordance with orientation of liquid crystals. Liquid crystals exhibit characteristics of abnormal response speed, elasticity, and dielectric constant at an abnormal temperature. For this reason, in LCDs, it is difficult to achieve normal display of an image under abnormal temperature conditions. In LCDs, which may be under abnormal temperature conditions, accordingly, a temperature sensor is used to sense the abnormal temperature condition, and thus, to allow the characteristics of liquid crystals to be compensated for in accordance with the sensed abnormal temperature condition.
Generally, such a temperature sensor is mounted to a driver or module arranged outside the liquid crystal panel of an LCD, so as to sense a temperature of the liquid crystal panel.
In fields where LCDs are used, in particular, aerospace fields where liquid crystal panels may be exposed to low temperature conditions, efforts have been made to heat a liquid crystal panel to cope with a low temperature condition.
Hereinafter, temperature dependency of liquid crystals and a conventional IPS mode LCD device will be described with reference to the annexed drawings.
FIG. 1 is a graph depicting the response characteristics of liquid crystals varying depending on a variation in temperature. FIG. 2 is a graph depicting a variation in the voltage-dependent transmittance of liquid crystals depending on a variation in temperature.
After observing respective response characteristics of liquid crystals at 0° C., 20° C., and −20° C., as shown in FIG. 1, it can be seen that the response time taken for the liquid crystals to be changed from a white state (transmittance of 100%) to a black state (transmittance of 0%) or the time taken for the liquid crystals to be restored from the black state to the white state is longer at a lower temperature of the liquid crystals because the response speed of the liquid crystals is lower at the lower temperature of the liquid crystals. In particular, at room temperature in which the temperature of the liquid crystals ranges from 0° C. to 40° C., it is possible to obtain a desired response speed range of the liquid crystals. At a temperature of −20° C., however, the liquid crystals exhibit a very low response speed. In this case, it is difficult to normally display a rapidly-varying image such as a video image.
Referring to FIG. 2, it can also be seen that the transmittance of liquid crystals exhibited at a constant application voltage varies depending on a variation in temperature such that a lower transmittance is obtained at a lower temperature (transmittance: −20° C.<0° C.<20° C.).
Generally, LCD devices display an image by varying the optical anisotrophy of liquid crystals having properties intermediate between liquids and solids, that is, the flowability of liquid and the optical properties of crystals. The liquid crystals are varied in transmittance and opto-electric characteristics depending on a variation in temperature. Accordingly, the temperature conditions, to which an LCD device is exposed, are an important factor in implementing desired display in the LCD device.
Therefore, although liquid crystals exhibit a variation in response speed depending on a variation in temperature, it would be expected that, if an LCD device is exposed to room temperature conditions of 0° C. to 40° C., or if the driving characteristics of the LCD device are compensated for through a temperature compensation means connected to the LCD device when the ambient temperature of the LCD device is an abnormally high or low temperature, the LCD device can exhibit normal driving characteristics.
FIG. 3 is a plan view illustrating a general IPS mode LCD device. FIG. 4 is a cross-sectional view taken along the line I-I′ of FIG. 3.
As shown in FIGS. 3 and 4, the general IPS mode LCD device mainly includes a lower substrate 10, an upper substrate 20, which faces the lower substrate 10, and a liquid crystal layer 30 sealed between the two substrates 10 and 20.
Gate lines 11 and data lines 12 are formed on the lower substrate 10 such that the gate lines 11 and data lines 12 extend horizontally and vertically, respectively, to cross each other, thereby defining pixel regions. A common electrode 13 and a pixel electrode 15 are formed on the lower substrate 10 at each pixel region such that the common electrode 13 and pixel electrode 15 are spaced apart from each other. For simplicity, the following description will be given only in conjunction with one pixel region.
A TFT is also formed on the lower substrate 10. The TFT includes a gate electrode 11a formed on the lower substrate 10 such that the gate electrode 11a is protruded from the gate line 11. The TFT also includes a semiconductor layer 18 formed over the entire surface of the lower substrate 10 including the gate electrode 11a such that the semiconductor layer 18 overlaps with the gate electrode 11a in a state in which a gate insulating film 14 is interposed between the gate electrode 11a and the semiconductor layer 18. The TFT further includes a source electrode 12a and a drain electrode 12b formed at opposite sides of the semiconductor layer 18, respectively. The source electrode 12a is protruded from the data line 12. The drain electrode 12b is spaced apart from the source electrode 12a by a predetermined distance, and is connected to the pixel electrode 15.
The common electrode 13 is spaced apart from the pixel electrode 15 by the predetermined distance, and is formed on the layer, on which the gate line 11 or data line 12 is formed, simultaneously with the formation of the gate line 11 or data line 12. In the illustrated case, the common electrode 13 is formed on the layer on which the gate line 11 is formed.
An insulating film 16 is also deposited such that the insulating film 16 is interposed between the data line 12 and the pixel electrode 15. The insulating film 16 is made of the same material as the gate insulating film 14. For example, the insulating film 16 is made of an inorganic insulating material such as SiNx or SiOx, or an organic insulating material such as acryl resin, polyimide, benzocyclobutene (BCB), or photo polymer.
A protective film 17 is then formed over the entire surface of the lower substrate 10 including the insulating film 16 and pixel electrode 15.
The common electrode 13 is coupled to a common line 19 to receive a voltage signal. When the voltage signal is applied to the pixel electrode 15 via the drain electrode 12b, an in-plane electric field is generated, thereby driving the liquid crystal layer 30.
On the other hand, on the upper substrate 20, a black matrix layer 21 is formed to block incidence of light to a region other than the pixel regions. Color filter layers 22 for reproducing R, G, and B color tones, and an overcoat layer 23 for planarizing color films of the color filter layers 22 are also formed on the upper substrate 20.
The illustrated case corresponds to the optical mode of a general IPS mode LCD device, that is, a normally black mode. Accordingly, when no voltage is applied, the illustrated IPS mode LCD device is maintained in a black state in which transmission of light is prevented.
When a voltage is applied to the pixel electrode 15 and common electrode 13 formed on the same substrate, an electric field is generated between the two electrodes 13 and 15. The liquid crystals in the liquid crystal layer 30 are oriented along the electric field.
As a result, internal light passes through the liquid crystal layer 30 along the oriented liquid crystals, so that white is displayed.
As mentioned above, the common electrode 13 and pixel electrode 15 are formed on the same layer in the lower substrate 10. The liquid crystals interposed between the lower substrate 10 and the upper substrate 20, which are joined together to define a certain space therebetween, are driven by an electric field generated between the common electrode 13 and the pixel electrode 15. In this case, since the liquid crystals have positive dielectric anisotrophy, they have characteristics wherein the longer axis of the liquid crystals are oriented in the direction of the electric field.
In an OFF state in which no in-plane electric field is applied to the common electrode 13 or pixel electrode 15, the orientation of the liquid crystals does not vary. However, in an ON state in which an in-plane electric field is applied to the common electrode 13 and pixel electrode 15, the orientation of the liquid crystals varies. That is, in the ON state, the liquid crystals are oriented while having a twist angle of about 45°, contrary to the OFF state.
In such a conventional IPS mode LCD device, a temperature sensor is arranged outside the liquid crystal panel of the LCD device, in order to sense an internal temperature of the liquid crystal panel. A temperature compensation means is also arranged outside the liquid crystal panel. Through this arrangement, the liquid crystal panel is maintained at a normal temperature even under abnormal ambient temperature conditions.
However, the above-mentioned conventional IPS mode LCD device has various problems.
That is, liquid crystals generally used in LCDs are sensitive to temperature, and do not perform a normal response under low or abnormally high temperature conditions. For this reason, it is necessary to conduct a compensation coping with such an abnormal temperature condition.
In particular, application of LCDs to aerospace fields associated with aircraft and spacecraft and military fields are currently increasing. For this application, it is necessary to secure reliability under severe low temperature conditions, as compared to other fields.
To this end, a heat line is used in LCDs which may possibly be exposed to abnormally low temperature conditions.
However, such a heater line, which is applied to an LCD device to secure reliability under low temperature conditions, is generally formed outside the liquid crystal panel of the LCD device or inside the liquid crystal panel. Where the heater line is formed inside the liquid crystal panel, the heater line may be formed over the entire surface of the liquid crystal panel or on a part of the liquid crystal panel which is not clearly defined. For this reason, an abnormal orientation of liquid crystals occurs at a region where the heat line is exposed, thereby causing undesirable leakage of light.
In particular, in IPS mode LCD devices, problems of difficulty in securing a desired aspect ratio and undesirable leakage of light may occur in accordance with the position of the heat line.